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Lifting bows with foresails
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Streamlines & swirls

Flow simulation: streamlines & leech vortices

Streams

 

This little study in CFD focuses on air flow around the sails, visualized with the help of streamlines. The streamlines are colored by the local flow speed to give an idea about the flow acceleration and deceleration.

 

The airflow around sails is complex. On the windward side air is decelerated so there is a higher pressure, and on the leeward side vice versa. This creates a problem at the ends, the top and the foot of the sail where the pressures tend to equalize. In consequence, towards the top of the sail air is bending upwards on the windward side and downwards on the leeward side. At the foot the opposite is true: air tends to escape or leak under the boom and/or the foot of the sail.

This results in crossflows at the leech where air is leaving the sails. The crossflows roll into larger vortices or swirls behind the sail. These vortices consume energy, forming easily 50% of the total air drag of a sailboat. The vortices also induce velocities over the sails themselves, much like the apparent wind is formed through boat movement and true wind - we are in a chicken-egg situation, where the vortices can be thought to bend the air flow, or we can also think that the vortices are formed because of the bent air. On the windward side of the vortice core, an upward component blends into the air flow, and on the leeward side a downward component influences the local flow.

If air wasn't invisible...

The computer simulation on the right shows how complex the airflow around sails can be. In this view from behind you can discern the vortices towards the top of the sails and those at the foot behind the clew of each sail.

WindLee

PressWindLee

Flow simulation on the surface of the sails, seen from the leeward side (left) and the windward side (right), colored with pressure (Cp).

On the lee side, see how the air is flowing up from the foot. In the upper part air flows almost perpendicular to the luff of the sail, with a little descending tendency. Red tones correspond to accelerated flow (diminished pressure, negative numbers on the scale). Acceleration is strongest in the front-top part of the genoa.

On the windward side, air is escaping under the boom in the bottom part and bending up towards the top part of the sail plan. Blue tones indicate higher pressure, turquoise means free wind pressure (zero on the pressure scale to the right.)

Telltales near the luff of the genoa witness about the theory: The one on the windward side is flowing higher up than the one behind the sail on the leeward side.

The very efficient sail plan is that of a 46-foot IMS racer-cruiser, apparent wind is 16 knots/22 degrees and heel 17 degrees. Take a look at the mainsail and the genoa, as photographed in the analyzed situation.

Smoke tests

Smoke tests in the windtunnel reveal the tip vortex behind the top of this Europe dinghy sail model. The sail is painted black to show the smoke more clearly (look at the video).

... at full scale

Look at this amazing shot courtesy Daniel Forster/Seahorse Magazine.

TipVortex

CFD simulations

These simulations reveal how the air flow is bent in the vicinity of sails. Why is the bending of the air of interest to us? It turns out that's how sails generate their power - the more air is bent, the more power. The power transferred to the boat through this bending follows Newton's law of action-reaction. If you want to read more, Tom Speer offers a clear and compact explanation.

In the first simulation a vertical layer of air is moved from the windward side to the leeward of the boat (will open in a new window).

In the second simulation a horizontal layer of air is moved up from deck level towards the top of the sail plan (will open in a new window).

In the third simulation you can see how the tip and the foot vortices develop behind a Europe dinghy sail. This is the "bad air zone" familiar to any racing sailor.

In the fourth simulation you can see how the wake develops behind the mainsail and the jib.

Vert layer

CFD = Computer Fluid Dynamics. CFD is a great tool for visualizing and explaining flow phenomena. While the latest flow software is very powerful and capable of calculating amazing things at astonishing accuracy, the old saying "garbage in, garbage out" is more true than ever. Besides of presenting the problem in a meaningful way, one needs lots of knowledge and experience to interpret the results correctly. Simulation through CFD is especially useful at giving qualitative information - when it comes to quantitative results or hard numbers, you have to be even more cautious when drawing conclusions about the merits of one design over another. Wind tunnel tests are needed to calibrate and validate the CFD code before reliable results are obtained.

With the power of modern CFD at the desktop, it is too easy to produce beautiful pictures with little connection to reality. Often these pictures are produced by flow experts with little sail-specific knowledge, and then interpreted by sail designers without sufficient understanding of the CFD tool, and as a result you get just that - pretty pictures. WB-Sails has used CFD as a part of sail development since late '80s and we have done many a wind tunnel test to take our CFD codes further and closer to reality. [back]

 

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